Add archival feature tagging on feature finish

When finishing a feature, add a tag with the feature name, prefixed with
the configured archive prefix. Example: a feature named foo would be
archived as archive/feature/foo. The tag is set immediately behind the
merge commit

Having such tags eases resumption of feature branches that are
prematurely finished. They also mark the end of each feature branch in a
way that's easier to find than searching for the final merge commit.

Add a flag to feature finish, -A, that enables archive tagging if it is
not on by default in the config.

Add a new boolean configuration option, gitflow.options.archivefeatures,
which if set true, archives features by default, without specifying -A
on the command line.

Add a new prefix in git flow init: gitflow.prefix.archivetag, with
default archive/

Add a summary message if branch was archive tagged.